When using inverter fed electrical motors, there are various optimum operating points depending on the desirable criteria. For example, in some instances it is desirable to find an operating point requiring as little electrical current as possible, for a given load torque requirement. This is also known as Maximum Torque per Ampere, MTPA. In other instances, it is desirable to find a point of maximum efficiency, ME, (or minimum input active power (kW), where the motor supplies a specific load torque with minimum losses in the machine. Other measurements are equally possible, such as maximum power factor (MPF), minimum input volt amperes (kVA), maximum rate of change of torque (MRT), and maximum torque per voltage (MTPV) control. System ME control where the overall efficiency of motor plus inverter system is of concern.
In the article entitled “Self-tuning of MTPA current vector generation scheme in IPM synchronous motor drives” by S. Bolognani et al., published in conjunction with a conference named EPE 2007, ISBN 9789075815108, page 1-10, it is presented a method of achieving MTPA. The article presents a drive prototype with a current vector generation scheme which can be self-adjusted in “quasi real-time” to the actual maximum torque per amps locus. The article presents a method to achieve an MTPA operating point by injecting a perturbation signal in the phase angle of current preference. By adjusting the current phase around an MTPA point, the MTPA is found where the absolute value of the current is at its minimum.
While it appears as if this method would find an MTPA point, varying the current angle to find an optimum is problematic. The torque of the motor is directly related to the current; hence, the variation as presented above, will generate a significant torque variation. Depending on the application, such a torque variation can be anything from undesirable to devastating for the particular application.
Consequently, there is a problem of increased torque ripple while finding an optimum operating point of motors.